C6 auto shift fine but no manual shift

I have a 80 Mustang with a 460 and a C6. It's a early model C6 with a Hastings shift kit installed. Long story short is it shifts fine in drive but I can't manually shift it. It just stays in 1st. what am I doing wrong. I've checked the linkage, and have 17 inches of vacuum at the modulator. It does not have a kick down lever or cable. Help !